The present invention relates to a method of producing a bonded wafer having very few crystal defects on and near the surface and a bonded SOI wafer.
SOI (silicon on insulator) has a buried oxide film (BOX: Buried OXide) as a insulator film right below a silicon layer that is to be a region for fabrication of device, and is expected to be a silicon material for high speed device with electric power saving performance. One of methods for producing a SOI wafer is a bonding method wherein two silicon single crystal wafers, one of which is to be a bond wafer (a substrate that is to be a SOI layer on which a device is fabricated), and the other of which is to be a base wafer (a substrate supporting the SOI layer) are bonded via a oxide film, and then thickness of the bond wafer is decreased to form a SOI structure. The method has an advantage that crystallinity of the SOI layer is excellent, and insulating property of BOX is high, but has a disadvantage that quality of the SOI layer is influenced much by quality of the bond wafer.
Specifically, it has been known that there exist micro crystal defects (Grown-in defects) such as COP (Crystal Originated Particles) that is a void type defect or the like in a silicon wafer produced according to Czochralski method, which adversely affects characteristics of the device such as oxide dielectric breakdown voltage. In order to solve the problem, there have been known that visible defects can be reduced by using, as a substrate for a bond wafer, a wafer wherein a CZ wafer is subjected to annealing in a hydrogen atmosphere or an epitaxial wafer wherein an epitaxial layer is formed on a CZ wafer (See Japanese Patent Application Laid-open (kokai) No. 9-22993 and Japanese Patent Application Laid-open (kokai) No. 9-260619).
However, two heat treatments, namely heat treatment such as hydrogen annealing or epitaxial growth and heat treatment for forming a buried oxide film, which may lead to increase of cost and lowering of through put.
In the case of the epitaxial wafer, haze (surface roughness) is generated on the surface of the epitaxial layer, or projection called mound is sometimes formed. They may cause bonding failure when the wafers are bonded. Accordingly, it is sometimes necessary to polish the surface of the epitaxial layer before bonding in that case.
On the other hand, crystal defects are reduced by hydrogen annealing only at layer quite near the surface (about 0.5 xcexcm from the surface), and thus, if a SOI wafer having a thickness more than the value is produced, an area where crystal defects are not reduced is exposed. Therefore, crystal defects in the whole SOI layer cannot be reduced, unless any measures are taken, for example, further hydrogen annealing is conducted after SOI wafer is produced. Furthermore, according to annealing with hydrogen, quartz tube, a boat made of SiC or the like are always etched, and contamination with metal impurities or the like are caused thereby.
Furthermore, when heat treatment is conducted in a hydrogen atmosphere, it is necessary to take out the wafer after replacing the atmosphere in the heat treatment furnace with nitrogen gas for safety. At that time, the surface of the wafer is locally etched with slight amount of oxygen and water vapor contained in nitrogen gas, which may degrading surface roughness such as haze or the like, which may lead to bonding failure when they are bonded.
Recently, it has been reported that there can be produced CZ wafer wherein Grown-in defects are significantly reduced if crystal is pulled with strictly controlling a growth rate and temperature gradient of solid-liquid interface while single crystal is grown according to Czochralski method. It can be easily expected that SOI wafer having few defects in SOI layer can be produced if such a wafer is used as a bond wafer. However, if the crystal is pulled under such significantly strict growing condition may naturally lead to lowering in yield, resulting in significant increase of cost for production.
On the other hand, single crystal produced according to FZ method has no COP defects as observed in CZ single crystal, but FZ crystal having a diameter more than 150 mm cannot be produced at commercial level. Although FZ crystal having a diameter of 200 mm can be produced at experimental level, there is no hope for producing a large diameter wafer having a diameter of 300 mm, 400 mm in the future.
The present invention has been accomplished to solve the above-mentioned problems. A main object of the present invention is to provide a SOI wafer that has a SOI layer having few crystal defects and high quality in high productivity, in high yield and with low cost by using a wafer wherein grown-in defects in a surface-layer part of silicon single crystal wafer produced by CZ method are eliminated or reduced effectively by heat treatment as a bond wafer of a bonded wafer.
To achieve the above mentioned object, the present invention provides a method of producing a bonded SOI wafer comprising bonding a bond wafer and a base wafer via an oxide film and then reducing thickness of the bond wafer, wherein a silicon single crystal ingot is grown according to Czochralski method, the single crystal ingot is then sliced to produce a silicon single crystal wafer, the silicon single crystal wafer is subjected to heat treatment in a non-oxidizing atmosphere at a temperature of 1100xc2x0 C. to 1300xc2x0 C. for one minute or more and continuously to a heat treatment in an oxidizing atmosphere at a temperature of 700xc2x0 C. to 1300xc2x0 C. for one minute or more without cooling the wafer to a temperature less than 700xc2x0 C. to provide a silicon single crystal wafer wherein a silicon oxide film is formed on the surface, and the resultant wafer is used as the bond wafer.
As described above, if the wafer produced according to Czochralski method is subjected to heat treatment in a non-oxidizing atmosphere at a temperature of 1100xc2x0 C. to 1300xc2x0 C. for one minute or more and continuously to a heat treatment in an oxidizing atmosphere at a temperature of 700xc2x0 C. to 1300xc2x0 C. for one minute or more without cooling the wafer to a temperature less than 700xc2x0 C. to provide a silicon single crystal wafer wherein a silicon oxide film is formed on the surface, and the resultant wafer is used as the bond wafer, a silicon single crystal wafer having high quality wherein Grown-in defects near the surface of the wafer that are harmful for fabrication of semiconductor device can be eliminated or decreased in short time can be used as a bond wafer, so that SOI wafer that has a SOI layer having few crystal defects and high quality can be produced in high productivity, in high yield with low cost.
The present invention also provides a method of producing a bonded SOI wafer comprising bonding a bond wafer and a base wafer via an oxide film and then reducing thickness of the bond wafer, wherein a silicon single crystal ingot is grown according to Czochralski method, the single crystal ingot is then sliced to produce a silicon single crystal wafer, the silicon single crystal wafer is subjected to heat treatment in a non-oxidizing atmosphere at a temperature of 1100xc2x0 C. to 1300xc2x0 C. for one minute or more and continuously to a heat treatment in an oxidizing atmosphere at a temperature of 700xc2x0 C. to 1300xc2x0 C. for one minute or more without cooling the wafer to a temperature less than 700xc2x0 C. to provide a silicon single crystal wafer wherein a silicon oxide film is formed on the surface, at least one of hydrogen ions and rare gas ions are implanted into the surface via a silicon oxide film of the wafer to form an ion implanted layer, and the resultant wafer is used as the bond wafer, which is then brought into close contact with the base wafer via the silicon oxide film of the bond wafer, followed by delamination at the ion implanted layer by heat treatment.
As described above, in method of producing a bonded SOI wafer, by using the method wherein the wafer produced according to Czochralski method is subjected to heat treatment in a non-oxidizing atmosphere at a temperature of 1100xc2x0 C. to 1300xc2x0 C. for one minute or more and continuously to a heat treatment in an oxidizing atmosphere at a temperature of 700xc2x0 C. to 1300xc2x0 C. for one minute or more without cooling the wafer to a temperature less than 700xc2x0 C. to provide a silicon single crystal wafer wherein a silicon oxide film is formed on the surface, at least one of hydrogen ions and rare gas ions are implanted into the surface via a silicon oxide film of the wafer to form an ion implanted layer, and the resultant wafer is used as the bond wafer, which is then brought into close contact with the base wafer via the silicon oxide film of the bond wafer, followed by delamination at the ion implanted layer by heat treatment (so called ion implantation delamination method), a silicon single crystal wafer having high quality can be used as a bond wafer, and surface condition of the SOI wafer after delamination is good, so that SOI wafer having excellent thickness uniformity can be produced by a relatively easy method.
In that case, the bond wafer delaminated at the ion implanted layer in the above-mentioned method of producing a bonded SOI wafer of the present invention can be used as a new bond wafer.
As described above, in the bond wafer delaminated at the ion implanted layer in the method of producing a bonded SOI wafer of the present invention, grown-in defects in zone at a depth of about 5 to 10 xcexcm or more from the surface are eliminated, and thickness of the thin film delaminated at the ion implanted layer is about one xcexcm at thickest, so that the bond wafer has denuded (low-defect) zone with sufficient depth, even though it is a wafer after delamination of a thin film. Accordingly, even if the surface of the wafer is polished for reuse, sufficient denuded zone remains. Therefore, if it is used as a new bond wafer, and bonded to the base wafer via the oxide film, and thickness of the bond wafer is decreased to produce a SOI wafer, it is not necessary to conduct further heat treatment of the bond wafer before bonding for elimination of grown-in defects. Thereby, a bonded SOI wafer having high quality can be produced efficiently.
Furthermore, the bond wafer delaminated at the ion implanted layer in the above-mentioned method of producing a bonded SOI wafer of the present invention can be used as a new base wafer.
At an inner part (a bulk part) than denuded zone near the surface of the bond wafer after delamination of a thin film, a lot of oxide precipitates are sometimes generated due to influence of heat treatment. In that case, if the wafer is used as a new base wafer, and bonded to the bond wafer via the oxide film, and thickness of the bond wafer is decreased to produce a SOI wafer, the SOI wafer having high performance in gettering of heavy metal impurities or the like can be obtained. In that case, even though a lot of oxide precipitates are generated in a bulk part, a surface-layer part is denuded zone as described above, so that oxide precipitates are never exposed on the surface of the base wafer, and there is no adverse effect to bonding with a bond wafer.
The above-mentioned non-oxidizing atmosphere is preferably argon, nitrogen or a mixed gas of argon and nitrogen.
Because, the atmosphere of argon, nitrogen or a mixed gas of argon and nitrogen can be easily handled and inexpensive.
The above-mentioned oxidizing atmosphere may contain water vapor.
As described above, if the oxidizing atmosphere contains water vapor, an oxidation rate is high, and defects can be eliminated efficiently in quite short time by injection of interstitial silicon. Since the oxide film formed on the bond wafer gets relatively thick, it is suitable for production of SOI wafer having a thick BOX.
In that case, the above-mentioned oxidizing atmosphere can be dry oxygen atmosphere or a mixed gas atmosphere of dry oxygen and argon or nitrogen.
As described above, if the oxidizing atmosphere is dry oxygen atmosphere or a mixed gas atmosphere of dry oxygen and argon or nitrogen, a growth rate of the oxide film is low, and thickness of the oxide film formed on the surface of the bond wafer after heat treatment can be made thin, and thus, it is suitable for production of the SOI wafer having thin BOX.
The thickness of the oxide film formed by the above-mentioned heat treatment in the oxidizing atmosphere is preferably 20 to 100 nm.
As described above, if the thickness of the oxide film formed by the above-mentioned heat treatment in the oxidizing atmosphere is more than 20 nm, COP at a surface-layer part of the bond wafer can be removed sufficiently. If the thickness is 100 nm or less, time necessary for the step can be short even in the case that the formed oxide film needs to be removed. Furthermore, in the case that the SOI wafer is produced according to the above-mentioned ion implantation delamination method, thickness uniformity of the SOI layer gets better, since an absolute value of the deviation in thickness of the oxide film on the surface gets small.
The oxide film can be previously formed on the surface of the wafer before the heat treatment in a non-oxidizing atmosphere.
If such an oxide film is previously formed, the surface of the wafer can be protected so that formation of thermal nitride film on the surface of the wafer due to heat treatment or surface roughness due to etching can be prevented. Therefore, bonding failure when the wafers are bonded can be prevented.
In that case, thickness of the thermal oxide film on the surface of the wafer after the above-mentioned heat treatment in the oxidizing atmosphere is preferably 300 nm or more.
As described above, if the thermal oxide film having a thickness of 300 nm or more is grown, COP on the surface of the wafer can be eliminated by reflow phenomenon of silicon oxide during growth of the oxide film even when the oxide film is previously formed on the surface of the wafer before conducting the heat treatment in the non-oxidizing atmosphere, so that COP on the surface of the wafer can be eliminated more surely.
A silicon single crystal ingot is preferably grown according to Czochralski method with controlling a cooling rate at 1150xc2x0 C. to 1080xc2x0 C. of the single crystal ingot to be 2.3xc2x0 C./min or more.
As described above, if a silicon single crystal ingot is grown according to Czochralski method with controlling a cooling rate at 1150xc2x0 C. to 1080xc2x0 C. of the single crystal ingot to be 2.3xc2x0 C./min or more, a size of grown-in defect gets small. Since the above-mentioned heat treatment is conducted to such a wafer, grown-in defects in a surface-layer part of the wafer can be eliminated or reduced more efficiently. Accordingly, a SOI wafer having a SOI layer with higher quality can be produced in high productivity.
In that case, it is preferable that a silicon single crystal ingot in which nitrogen is doped is grown according to Czochralski method.
As described above, if a silicon single crystal ingot in which nitrogen is doped is grown according to Czochralski method, the size of grown-in defect becomes smaller by nitrogen doping. Further, and the heat treatment is conducted thereto, and thus grown-in defects in a surface-layer part of the wafer can be more efficiently eliminated or removed. Accordingly, a SOI wafer having a SOI layer with higher quality can be obtained in high productivity.
In that case, when growing silicon single crystal ingot in which nitrogen is doped according to Czochralski method, the concentration of nitrogen doped in the single crystal ingot is preferably 1xc3x971010 to 5xc3x971015 atoms/cm3.
Because, 1xc3x971010 atoms/cm3 or more is preferable in order to suppress growth of grown-in defects sufficiently, and 5xc3x971015 atoms/cm3 or less is preferable in order not to prevent formation of single crystal of silicon single crystal.
Furthermore, when the silicon single crystal ingot is grown according to Czochralski method, the concentration of oxygen contained in the single crystal ingot is preferably 18 ppma (JEIDA: Japan Electronic Industry Development Association) or less.
If oxygen concentration is low as described above, growth of crystal defects can be suppressed further, and formation of oxide precipitates at a surface layer can also be prevented.
A bonded SOI wafer produced according to the method of the present invention is, for example, a bonded SOI wafer wherein a SOI layer consists of CZ silicon single crystal wafer, thickness of the SOI layer is 5 xcexcm or less, and 1.3 number/cm2 or less of COP having a size of 0.09 xcexcm or more exist at any region in depth direction of the SOI layer.
As described above, in the bonded SOI wafer of the present invention, there exist very few COP at any region in depth direction of SOI layer, even if thickness of SOI layer is more than 0.5 xcexcm. Furthermore, SOI wafer of the present invention does not need to be subjected to hydrogen anneal or the like after production of SOI wafer, and thus productivity is also high.
According to the present invention, it is possible to eliminate void defects in deeper region efficiently compared with conventional methods, and therefore SOI layer with high quality can be formed. Furthermore, since heat treatment in non-oxidizing atmosphere and heat treatment in oxidizing atmosphere can be conducted in the same batch, the number of the steps in production of SOI does not increase, and thus cost therefor does not increase either. Furthermore, since heat treatment can be conducted without using hydrogen, heat treatment can be conducted with no danger of contamination from the furnace due to hydrogen and explosion. Furthermore, since CZ wafer is used, it can be applied to a wafer having a large diameter as 300 mm or more.